The Neogene desertification of Northern Africa and its palaeoenvironments

The present-day hyperarid Sahara is the largest hot desert in the world, but its development in deep time is largely unknown. The inception of persistent Northern African aridification has been variously linked to the onset of Northern Hemisphere glaciation at the beginning of the Pleistocene. However, eolian bedforms and dust records, reinforced by modelling studies, indicate transient periods of aridity have started North Africa already in the latest Miocene during times of Northern Hemisphere cooling. For the globally warm Pliocene epoch intermittent aridity is suggested for the western Sahara by dust records from the Canary Islands. In contrast, onshore evidence of contemporary North African climate is missing, because of the lack of well-dated Neogene sections in the Sahara.

Here we present a thoroughly sedimentological, paleoenvironmental and geochronological investigation of the Neogene continental Wadi El Natrun Formation in the northern fringe of eastern Sahara (Wadi El Natrun, northern Egypt). We differentiate seven sedimentological facies types, each distinguished by characteristic palaeobiota. These are in an ascending order: braided river and overbank facies, which contain abundant fluvial and riparian vertebrates; a lacustrine black shale facies rich in phytoclasts; a lacustrine limestone facies with characean algae; a marine limestone facies with planktonic foraminifera and marine molluscs; a playa lake facies with a thalassosaline ostracod and gastropod fauna; an ephemeral stream facies with Skolithos and Ophiomorpha ichnofabrics; and finally an eolian dune facies with polydomic ichnofabrics.

These changes in sedimentological facies indicate the long-term temporal progression of desertification in Northern Africa well before the Quaternary.